Electrochemical measurements are used for qualitative analysis of ions, molecules contained in a solution and the like. Working electrodes are immersed in a sample solution containing a test substance, and when the test substance is electrochemically reacted on the working electrodes, a value of an electric current flowing through the working electrodes is measured, whereby the test substance in the sample solution is detected.
As the electrochemical detector used for electrochemical measurements, those in which two comb-shaped working electrodes are combined are widely used (for example, Patent Document 1).
FIGS. 11A, 11B show an example thereof. FIG. 11A is a plan view of an electrochemical detector in which two comb-shaped electrodes are combined, and FIG. 11B is a cross-sectional view of FIG. 11A taken along line A-A′. In the electrochemical detector shown in FIGS. 11A, 11B, comb-shaped working electrodes 802 and 803 are formed on an insulating substrate 801 so as to engage with each other. An electrochemical measurement is performed in the following manner. First, droplets of a sample solution 200 containing a test substance are dropped on the electrochemical detector. Then, when different potentials are applied respectively to the working electrodes 802 and 803, the test substance is oxidized on one electrode (for example, 802), and then reduced on the other adjacent electrode (for example, 803) to return to the original substance. As a result, so-called redox cycling in which oxidation and reduction are repeated between the two electrodes 802 and 803 occurs.
FIG. 12 schematically shows redox cycles generated in the electrochemical detector in which the two comb-shaped electrodes are combined. A reductant Red, which is the test substance in the sample solution, is oxidized in the working electrode 802 to become an oxidant Ox, which is then oxidized in the adjacent other working electrode 803 to return to the original reductant Red. One reductant repeats oxidation and reduction, whereby the apparent quantity of an electric current flowing through the working electrodes is increased following the oxidation-reduction reactions.
Therefore, if an electrochemical measurement is performed using such electrodes, the test substance in the sample solution can be detected with a high sensitivity. In particular, the higher the density between the adjacent two electrodes, where the oxidation-reduction reactions are generated, namely, in FIGS. 11A and 11B, as the width of the electrodes 802, 803 and a space between them become smaller, the number of occurrences of redox cycling per unit time is increased, so that a highly sensitive electrochemical measurement is enabled.
Recently, an electrochemical measurement using the electrochemical detector is also applied to a biosensor for measuring a protein such as an antigen in a living body. In this case, in order to detect the protein, for example, a complex between the protein and an enzyme-labeled antibody is formed. Further, the enzyme label of the complex is reacted with a substrate to produce a substance carrying out redox cycling (a redox species). Since the amount of the redox species produced is proportional to the amount of the protein, the produced redox species is detected by the electrochemical measurement, whereby the concentration of the protein can be indirectly measured.